The present invention relates to a current transducer of the closed-loop type with a switched mode amplifier.
Certain known current transducers have a magnetic field detector and a magnetic circuit designed to surround a primary conductor through which a current to be measured flows, and to concentrate the magnetic field generated by the current flowing in the primary conductor. This magnetic field, which forms an image of the current to be measured, is sensed by the magnetic field detector. Open-loop current transducers generate a measurement signal from the magnetic field detector, whereas in closed-loop transducers the signal from the magnetic field detector is used to generate an opposing magnetic field by means of a compensation coil (also called secondary coil) that seeks to cancel the magnetic field generated by the primary conductor. In other words, closed-loop sensors have a feed-back signal that seeks to annul the magnetic field generated by the primary conductor. The current that flows in the compensation coil represents an image of the current to be measured and may thus be used to supply the current measurement signal. Closed-loop current transducers are generally more costly than open-loop current transducers for a given current measurement range, but have the advantage of greater sensitivity and accuracy. Magnetic field detectors employed in current transducers may be Hall effect detectors, magneto-resistive detectors or fluxgate detectors. Fluxgate detectors are generally more sensitive and accurate than Hall effect detectors.
In closed-loop transducers, the signal generated by the magnetic field detector needs to be processed and amplified in order to drive the compensation coil and provide the measurement signal output. In certain conventional current transducers, linear amplification circuits are employed, however linear amplifiers consume, dissipate and waste a relatively high amount of energy. For certain applications, particularly applications that require large current measurement as found in electrical motors for railways or in other high current applications, the transducer amplification circuit requires cooling means and a large volume in order to evacuate excess heat at the high operational temperatures.
Heat generation may be reduced by using more efficient amplifiers, in particular switched mode amplifiers such as the well-known H bridge amplifier circuits used in known closed-loop current transducers. An example of a known H bridge amplifier for a current transducer is described for example in WO 9836281. This type of amplifier is however generally more costly than linear amplifiers of comparable power, may require a split compensation coil, and generates a differential output voltage that is less useful or less desirable than an absolute measurement output (i.e. with respect to ground (0 Volts)).